The present invention relates to improved constructions for transformers and reactors having windings of tape-formed conductor material, the constructions reducing additional losses in the windings by controlling the magnetic flux at the ends of the windings.
The magnetic leakage flux primarily passing axially through the windings and in the gaps between the windings of a transformer or reactor tends to deflect at the ends of the windings and partially enter the core legs and therefore the flux also acquires a radial component. This component tends to become most pronounced at the corners of the cross-section of the winding which are nearest to the core leg that is surrounded by the winding. In conventional windings where the current is conducted in discrete conductors having a small extension in the radial and axial directions, a radial component of the magnetic flux also exists, but this component is not so heavily concentrated in a small region as is the case with tape windings.
In windings having conductors of tape-formed conductor material, especially in windings having a large radial extension, the strongly concentrated and radially directed leakage flux at the region about the ends of the windings will generate considerable additional losses caused by the eddy currents in the tapes which are induced by the radial component of the magnetic leakage flux. These losses limit the applications of tape-formed conductor material in transformer and reactor windings, although the use of such conductor material results in great advantages of various kinds. In conventional windings according to the above, admittedly eddy currents are induced and these currents cause losses through the radial component in the leakage flux, but these losses are limited to an acceptable level by choosing a conductor having a sufficiently small axial extension.
U.S. Pat. No. 4,060,784 to Fergestad illustrates how previously attempts have been made to eliminate the effect of the radial component of the leakage flux at the ends of transformer windings having tape-formed conductors. In this patent, leakage flux is controlled by means of plates 22 of a magnetically conductive material located between the conductor tapes. The magnetically conductive plates may extend throughout the whole winding from one end surface to the other, but as an alternative, the plates may be located only within one region nearest the ends of the windings as shown in FIGS. 4 and 5, the plates being are situated within the very winding.
However, by positioning the magnetically conductive material inside the winding parallel to the conductor tape 21, the diameter of the winding will increase and a deteriorated fill factor will result. The increased diameter of the winding will therefore require a longer iron core, a larger transformer tank and more oil. Thus, both an increased total volume and a higher total weight of the transformer will result which are considerable drawbacks which will increase with the size of the transformer.
In addition, since the flux-controlling plate 22 terminates at the end surface of the winding and the flux strives to deflect radially at the ends of the winding, the deflection of the flux, which in the absence of a controlling plate inside the winding starts at a distance from the winding end and successively increases towards the winding end, will be concentrated in a small region at the very end of the winding. This concentration will considerably increase the additional losses in a narrow zone at the very end surfaces of the winding and the temperature will increase in this zone to a considerably greater extent than what would have been the case had there been no flux-controlling plates inside the winding. Theoretical calculations performed also show that this is the case.
Furthermore, by introducing plates of such material inside the windings, the magnetic coupling is reduced between the windings and therefore the functions of the transformer are lessened.
British Patent Specification 990,418, published Apr. 28, 1965, illustrates another principle for controlling the radial component of the leakage flux for the purpose of reducing the additional losses in the winding ends when using taped-formed conductor material. This patent discloses that shields 20 of electrically conductive material are placed between the core legs and the inner winding as well as outside the outer winding. The shields extend axially outside the winding ends and the eddy currents in the shields, caused by the radial component of the leakage flux, generate a flux around the shields which tends to straighten the total leakage flux. However, in this device the inner shield occupies such a space inside the windings that all the windings have to be given an enlarged diameter which thereby results in a larger volume for the windings.
French Patent Specification 1,557,420 discloses a device in transformers for straightening the leakage flux passing between and through the windings so as to avoid additional losses at the ends of the windings. Outside the ends of the windings are arranged magnetic regions 8, 9 which are constructed from ferro-magnetic strips which are wound into a coil. The strip may be connected to the winding conductor in several different ways.
In transformers for great power where powerful leakage fluxes occur, the above-disclosed solution will with great probability not be sufficient. The flux density at the inner corners of the inner winding will become unallowably great despite the magnetic regions because the leakage flux will at least be partly deflected radially before reaching the ends of the winding. The proposed solution with only magnetic regions outside the winding ends is therefore not sufficient, especially for large units having great power with large leakage flux density.